As is known in the art, to reduce exhaust emissions, particularly, NOx emissions, internal combustion engines, particularly diesel engines, use selective catalytic reduction systems. In such reduction systems, a reductant, typically ammonia (NH3), is used as the reducing agent. More particularly, in such a system, regulated emissions, such as nitrogen oxides, NOx, can be reduced in an oxygen-rich environment to nitrogen and water over a catalyst when a reducing agent, such as ammonia, is added. While it is desirable to consume all of the added reductant in the process of reducing the NOx, an un-consumed portion of the reductant; i.e., slip, is produced. This slip is in effect a waste of the reductant.
One method suggested for regulating reductant slip is to use a reductant sensor; i.e., an ammonia sensor when ammonia is used as the reductant. In such case, the ammonia sensor is located downstream of the catalyst. The detected ammonia concentration is compare with a fixed upper threshold value. This comparison generates a correction signal that is used to control the metering of ammonia upstream of the catalyst. Allegedly, by regulating actual ammonia slip to the upper threshold value, a certain NOx reduction is obtained. Such a system is disclosed in U.S. Pat. No. 5,369,956.
Another method for regulating NOx emissions and reductant slip is to use an after-catalyst NOx sensor to detect NOx concentration. Control of NOx emission is achieved by varying reductant injection until the level or quantity of NOx, as measured by the sensor, falls within an acceptable limit. The amount of reductant injected to keep NOx emissions within the acceptable limit needs to be balanced with an ammonia slip limit. This can be measured and controlled by an after-catalyst ammonia sensor. Such a system is disclosed in U.S. Pat. No. 5,233,934.
Alternatively, ammonia slip can be calculated and controlled using an algorithm. Such a system is disclosed in U.S. Pat. No. 4,751,054.
Applicants have recognized that, in general, as maximum NOx conversion is approached with increasing the reductant, e.g., ammonia addition, i.e., increasing NH3/NOx molar ratio, ammonia starts to slip. After maximum NOx conversion is attained, ammonia slip increases more rapidly with increasing NH3/NOx. For example, if ammonia slip is regulated to a constant concentration value, as by sensing the amount of ammonia downstream of the catalyst, an ammonia setting high enough for sufficient NOx conversion at high NOx feed gas levels is likely excessive for low NOx feed gas levels (i.e., at low torque demands), thereby wasting ammonia.
Conversely, a setting at minimum detectable ammonia concentration is likely insufficient to provide high NOx conversion at high NOx feed gas levels (i.e., at high torque demands). Further, intermediate settings may still be insufficient to provide high enough NOx conversion at high NOx feed gas levels. Thus, prior approaches cannot achieve high NOx conversion with minimal ammonia slip, particularly for vehicle engine where NOx concentration levels varies widely and quickly. In other words, because a catalyst experiences widely varying levels of engine NOx, controlling to ammonia slip concentration results in widely varying and less than optimum NOx conversion efficiency.
To overcome drawbacks of prior approaches, the inventors of the present invention disclose a system and method for controlling reductant injection into a catalyst disposed in the exhaust of an internal combustion engine. The method and system determine a reductant fraction representative of the fractional amount of the reductant used in the catalyst (i.e., the amount of reductant used in the reaction in the catalyst) and the amount of the reductant passing through the catalyst. The method and system vary the amount of reductant injection in accordance with both the determined reductant fraction and the amount of reductant passing from the catalyst.
With such method and system, the amount of reductant is used efficiently when the engine is under either a high torque demand or low torque demand condition.
In a preferred embodiment, when the amount of reductant passing from the catalyst (i.e., the post-catalyst reductant concentration) is less than an allowable fraction and level, the amount of reductant is increased. If the increase in reductant leads to no increase in NOx conversion, the reductant is reduced. When the post-catalyst reductant concentration is greater than an allowable fraction, the amount of reductant is decreased. Further, when the post-catalyst reductant concentration is greater than an allowable concentration, the amount of reductant is decreased.
An advantage of the present invention is that because reductant slip is based on two limits, a post-catalyst reductant concentration and the reductant fraction, the values of the limits can be set at less restrictive values. In this way, reductant slip is maintained over the operating conditions of the engine while maintaining high NOx conversion.
Another advantage of the present invention is that in addition to controlling reductant slip based on the two limits, the amount of reductant is increased only when it leads to an improvement in NOx conversion in the catalyst. In this way, reductant slip is allowed to incur the two limits, but only when NOx is being reacted in the catalyst by incurring the limits. The result is a lesser amount of reductant slip without harming NOx conversion.
The above advantages, other advantages, and features of the present invention will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings.